Fuel cell with a large exchange surface area

ABSTRACT

A support wafer made of silicon wafer comprising, on a first surface a porous silicon layer having protrusions, porous silicon pillars extending from the porous silicon layer to the second surface of the wafer, in front of each protrusion. Layers constituting a fuel cell can be formed on the support wafer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming a fuel cell.

2. Discussion of the Related Art

Integrated fuel cells formed on silicon wafer are known. A drawback ofthe usual structures lies in the limited exchange surface area betweenthe active component (for example hydrogen) arriving from aperturesthrough the silicon wafer and an active layer of the cell.

SUMMARY OF THE INVENTION

The present invention provides a new fuel cell structure and a suitablesupport wafer.

The present invention also provides a method for manufacturing a fuelcell support wafer and a method for manufacturing a fuel cell.

An embodiment of the present invention provides a method for forming asupport wafer from a silicon wafer, comprising the steps of forming,according to a TGZM method, aluminum-silicon pillars crossing thesilicon wafer; forming cavities in a first surface of the silicon waferoutside of the areas corresponding to the pillars; diffusing a dopantmaterial such as boron into said first surface of the wafer to formdoped silicon portions; and performing an electrolysis of the siliconwafer, whereby said aluminum-silicon pillars and said doped siliconportions are turned into porous silicon.

Another embodiment of the present invention provides a method forforming a fuel cell on a support wafer as above, and further comprisingthe steps of performing a conformal deposition, on said first surface ofthe wafer, of a first conductive layer intended to be connected to ananode collector; forming through openings in the first conductive layer;successively performing conformal depositions of a first catalyst layer,of an electrolyte layer, and of a second catalyst layer on the firstconductive layer; performing a conformal deposition, on the secondcatalyst layer, of a second conductive layer intended to be connected toa cathode collector; and forming through openings in the secondconductive layer.

Another embodiment of the present invention provides a support wafermade of silicon wafer comprising, on a first surface a porous siliconlayer having protrusions, porous silicon pillars extending from theporous silicon layer to the second surface of the wafer, in front ofeach protrusion. Another embodiment of the present invention provides afuel cell formed on the above support wafer.

According to an embodiment of the present invention the fuel cellcomprises, on the first surface of the support wafer, a first conductivelayer intended to be connected to an anode collector and exhibitingthrough openings above the porous silicon portions, a superposition of afirst catalyst layer, of an electrolyte layer, and of a second catalystlayer on the first conductive layer, as well as a second conductivelayer intended to be connected to a cathode collector and exhibitingthrough openings.

The foregoing and other objects, features, and advantages of the presentinvention will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2A, and 2B are cross-section views illustrating firstsuccessive steps of a method for forming porous silicon pillars througha silicon wafer;

FIG. 2A is a cross-section view of an electrolysis device in which thelast step of a porous silicon pillar forming method is performed;

FIG. 3B is a cross-section view of a final structure obtained at the endof the porous silicon pillar forming method; and

FIGS. 4A to 4H are cross-section views of structures obtained onsuccessive steps of a method for forming a fuel cell on a support waferformed according to the method of the present invention.

DETAILED DESCRIPTION

For clarity, the same elements have been designated with the samereference numerals in the different drawings and, further, as usual inthe representation of semiconductor structures, the various drawings areout of scale.

The present invention uses a silicon wafer, in which porous siliconportions having the shape of through pillars and exhibitingsubstantially rectilinear walls perpendicular to the main wafer surfaceshave been formed. The method comprises the initial forming of thethrough pillars with aluminum-silicon according to a “temperaturegradient zone melting” method (TGZM), followed by the electrolyticturning of the silicon-aluminum pillars into porous silicon pillars.

In an initial step, illustrated in FIG. 1, an aluminum layer isdeposited on a silicon wafer 1 and etched to keep aluminum portions 10,11, and 12.

At the next step, illustrated in FIG. 2A, the previously-obtainedstructure is placed in a heating enclosure. It is seen to it that atemperature gradient between upper surface 1 f of the silicon waferwhere aluminum portions 10, 11, and 12 are laid and lower surface 1 f ofthe wafer is present in the heating enclosure. For this purpose, a heatsource 15 is placed on the rear surface side of the wafer and a blackbody 16 is placed on the upper surface side of the wafer. The gradientfor example is of 100 degrees per cm, which corresponds to a temperaturedifference of approximately 3° C. for a 300-μm wafer. Further, thetemperatures of the different portions of the heating enclosure, or morespecifically of the wafer, are greater than the melting temperature ofaluminum and smaller than the melting temperature of silicon. Atemperature on the order of 1,280° C. is, for example, selected.

Aluminum portions 10, 11, and 12 start melting and their melting causesa melting of the neighboring silicon. Under the surface of aluminumportions 10, 11, and 12, liquid aluminum-silicon areas 20, 21, 22 form.The temperature of lower surface 1 r of the wafer being greater thanthat of upper surface 1 f, the silicon melting, induced byaluminum-silicon areas 20, 21, 22, is preferentially performed on thelower surface side. Thereby, liquid aluminum-silicon areas 20, 21, 22progressively extend to lower surface 1 r, until reaching the latter.During the “progress” of liquid aluminum-silicon areas 20, 21, 22, analuminum-silicon precipitate forms on the side of upper surface 1 f. Alloccurs as if aluminum portions 10, 11, 12 were moving through siliconwafer 1 from the upper surface to the lower surface by leaving on theirway solid aluminum-silicon portions.

In a next step, illustrated in FIG. 2B, wafer 1 is removed from theheating enclosure. Aluminum-silicon portions 30, 31, 32 super-saturatedwith aluminum and covered with alumina are present on the lower surfaceof wafer 1. Aluminum-silicon pillars 40, 41, 42 are formed across theentire thickness of wafer 1.

It should be noted that, in the heating enclosure, the forming ofaluminum-silicon carries on as long as aluminum portions 10, 11, 12 havenot totally reacted. In the case where the aluminum quantity of eachportion 10, 11, and 12 is greater than the quantity necessary to form analuminum-silicon pillar across the entire thickness of wafer 1,aluminum-silicon portions 30, 31, 32 keep on reacting with silicon, byextending laterally at the surface of rear surface of wafer 1. In thecase where it is not desired to have such a lateral extension, a controlsystem detecting the forming of aluminum-silicon portions 30, 31, 32 maybe used.

Aluminum-silicon portions 30, 31, 32 are then eliminated, for example,by polishing.

In what follows, the surface of wafer 1 on which aluminum has initiallybeen deposited will be called front surface 1 f and the opposite surfacewill be called rear surface 1 r.

At the next step, illustrated in FIG. 3A, an electrolysis of thepreviously-obtained structure is performed. The electrolysis devicecomprises two hydrofluoric acid baths 50 and 51, in which are plungedplatinum electrodes 52 and 53, respectively connected to negative andpositive terminals of a supply voltage. The hydrofluoric acid of baths50 and 51 is regularly renewed through inlets E1 and E2 and outlets S1and S2 of baths 50 and 51.

It should be noted that another electrolysis device may be used. One ofthe wafer surfaces may for example be placed in contact with ahydrofluoric acid and another surface in contact with a metal electrode.

In this example, front surface 1 f of wafer 1 is in contact with bath 50connected to the negative terminal. Previously-formed aluminum-siliconpillars 40, 41, 42 progressively turn into porous silicon, where theportions in contact with bath 50 transform first.

It should be noted that the wafer direction is of no importance. Frontsurface 1 f of the wafer could be in contact with bath 51 connected tothe positive terminal.

It should be noted that to obtain porous silicon across the entirethickness of the silicon wafer, the “etching” of the aluminum-siliconportions should be performed smoothly with a low electrolysis currentand hydrofluoric acid concentration.

As a non-limiting indication, for a 300-μm wafer, baths exhibiting a 30%hydrofluoric acid concentration and a 200-mA/cm² electrolysis currentdensity may be used.

As illustrated in FIG. 3B, after the electrolysis step, porous siliconpillars 60, 61, and 62 crossing silicon wafer 1 are obtained.

It should be noted that the walls of pillars 60, 61, and 62 aresubstantially rectilinear across the entire wafer thickness, exceptclose to the front and rear surfaces where they are more flared.

The above method enables forming very precisely delimited porous siliconpillars. Indeed, it is possible to form pillars exhibiting a diameter,or a thickness, of a few micrometers through a wafer that may have athickness greater than a few millimeters, or even a few centimeters.

The present invention uses the above method to form a fuel cell support,as described hereafter in relation with FIGS. 4A to 4H.

In an initial step, illustrated in FIG. 4A, aluminum-silicon pillars101, 102, 103 are formed in a silicon wafer 100 according to the TGZMmethod previously described in relation with FIGS. 2A and 2B.

A protection layer is then deposited on silicon wafer 100 and etched tokeep protection blocks 111, 112, and 113 above aluminum-silicon pillars101, 102, and 103. The protection blocks are for example formed ofsilicon oxide.

It should be noted that the protection blocks may be placed on any oneof the wafer surfaces.

According to an alternative embodiment, the aluminum-silicon portionscovered with alumina present at the rear surface of the silicon waferafter forming of pillars 101, 102, and 103 according to the TGZM methodare used as protection blocks.

At the next step, illustrated in FIG. 4B, the upper portion of wafer 100is etched to form cavities 120, 121, 122, and 123 betweenaluminum-silicon pillars 101, 102, and 103 and on the side of pillars101 and 103. Protection blocks 110, 111, and 112 are then eliminated.

At the next step, illustrated in FIG. 4C, a surface layer saturated withboron 130 is deposited on the previously-obtained structure. The waferis then placed in a heating enclosure to diffuse the boron into theupper portion of the wafer.

At the next step, illustrated in FIG. 4D, surface layer 130 iseliminated. The upper portion of wafer 100 is then entirely P-typedoped. Cavities 120, 121, 122, and 123 are surrounded with doped siliconportions 140,141, 142. Further, a thin doped silicon layer 143 is formedunder cavities 120 to 123.

According to an alternative embodiment of the steps illustrated in FIGS.4A to 4D, it is possible to cover silicon wafer 100 with a surface layersaturated with boron, to diffuse the boron into the upper portion of thewafer to form a P-type doped silicon layer, then to form, by etching,cavities in the doped silicon layer. The depth of the cavities must thenbe smaller than the thickness of the doped silicon layer to obtain astructure identical to that shown in FIG. 4D.

At the next step, illustrated in FIG. 4E, an electrolysis of thepreviously-obtained structure is performed in a device such as thatshown in FIG. 3A. At the end of this electrolysis, aluminum-siliconpillars 101, 102, 103, thin doped silicon layer 143, and doped siliconportions 140, 141, and 142 are turned into porous silicon. Pillars 150,151, and 152, a thin layer 153, and portions 154, 155, and 156 made ofporous silicon are then obtained.

It is possible to diffuse into the wafer a dopant material other thanboron. The doped silicon portions obtained after diffusion must be ableto turn into porous silicon during the electrolysis step.

At the step illustrated in FIG. 4F, a conformal deposition of aconductive layer 160 on the previously-obtained structure is performedabove porous silicon portions 154, 155, 156 and thin layer 153. Throughopenings are then formed in conductive layer 160.

At the step illustrated in FIG. 4G, conformal depositions of a catalystlayer 170, of an electrolyte layer 171, and of a catalyst layer 172 onconductive layer 160 are successively performed.

At the step illustrated in FIG. 4H, a conformal deposition of aconductive layer 180 is performed on catalyst layer 172. Throughopenings are then formed in conductive layer 180. Several successiveetchings of layers 180, 172, 171, and 170 are then performed to form anaccess 190 to conductive layer 160.

Conductive layer 160 forms an anode collector, conductive layer 180forming a cathode collector. Pillars 150, 151, and 152, thin layer 153,and porous silicon portions 154, 155, and 156 form supply lines of a gassuch as hydrogen.

In operation, the hydrogen is “decomposed” at the level of catalystlayer 170 to form, on the one hand, H⁺ protons which direct towardselectrolyte layer 171 and, on the other hand, the electrons which directtowards anode connector 160. The H⁺ protons cross electrolyte layer 171to reach catalyst layer 172 where they recombine with oxygen arrivingfrom the top of the cell through the openings formed in cathodeconductive layer 180 and electrons arriving through cathode collector172. A positive voltage is then obtained on cathode collector 180, onthe oxygen side, and a negative voltage is obtained on anode collector160.

An advantage of the cell shown in FIG. 4H, and formed according to theabove-mentioned method, is that it exhibits a very wide surface area ofexchange between the hydrogen and catalyst layer 170. Indeed, thehydrogen arriving through the pillars 150-152 is distributed onsubstantially all the surface of the catalyst layer 170 due to thepresence of thicker portions 154-156 of porous silicon that act asshower head.

It should further be noted that the first part of thepreviously-described method provides a new support wafer structure,shown in FIG. 4E.

The forming of such a structure by means of known methods for formingand etching porous silicon would comprise the performing of a firstelectrolysis of a silicon wafer to form through pillars by using masksprovided with openings, followed by a second electrolysis to turn intoporous silicon an upper portion of the wafer, then the etching ofcavities in the upper portion of the porous silicon wafer. Such a methodwould require long hours of electrolysis, and would not enable formingnarrow pillars. Further, porous silicon etch methods are very difficultto implement and many fractures of the porous silicon can be observed.

The method of the present invention enables, in addition to throughpillars, forming porous silicon portions of various shapes. Thedefinition of these various shapes may be performed by means of dopingand etch methods which are easy to implement. Further, the poroussilicon being formed last, the wafer, and especially the porous areas,are not weakened by subsequent shape definition etchings.

Of course, the present invention is likely to have various, alterations,improvements, and modifications which will readily occur to thoseskilled in the art. In particular, those skilled in the art may deviseother structures of fuel cell supports manufactured according the methodof the present invention. Hydrogen ducts comprising through poroussilicon pillars and a thin porous silicon layer formed on one of thesurfaces of a silicon wafer on which are placed the different layersforming the fuel cell may for example be provided in a silicon wafer.Further, the actual forming of a fuel cell on such supports may beperformed according to various methods.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

1. A non-porous support wafer made of silicon wafer comprising, on afirst surface a porous silicon layer having protrusions, porous siliconpillars extending from the porous silicon layer through the non-poroussupport wafer to the second surface of the non-porous support wafer, infront of each protrusion.
 2. A fuel cell formed on the support wafer ofclaim
 1. 3. The fuel cell of claim 2, comprising, on the first surfaceof the non-porous support wafer, a first conductive layer intended to beconnected to an anode collector and exhibiting through openings abovethe porous silicon portions, a superposition of a first catalyst layer,of an electrolyte layer, and of a second catalyst layer on the firstconductive layer, as well as a second conductive layer intended to beconnected to a cathode collector and exhibiting through openings.
 4. Afuel cell, comprising: a non-porous support wafer fabricated from asilicon wafer, the non-porous support wafer including a porous siliconlayer having a plurality of protrusions on a first surface of thenon-porous support wafer, the non-porous support wafer further includinga plurality of porous silicon pillars extending from the porous siliconlayer to a second surface of the non-porous support wafer opposite thefirst surface, each porous silicon pillar being located in a region ofthe non-porous support wafer corresponding to a protrusion; first andsecond catalyst layers, the first catalyst layer adapted to be connectedto an anode collector and the second catalyst layer adapted to beconnected to a cathode collector; an electrolyte layer disposed betweenthe first and second catalyst layers; a first conductive layer disposedbetween the porous silicon layer and the first catalyst layer, the firstconductive layer having a plurality of through openings adapted to allowpassage of a first gas from the porous silicon pillars and the poroussilicon layer to the first catalyst layer; and a second conductive layerdisposed on the second catalyst layer, the second conductive layerhaving a plurality of through openings adapted to allow passage of asecond gas to the second catalyst layer.
 5. The fuel cell of claim 4,wherein the porous silicon layer has a plurality of cavities formed in asurface thereof, adjacent protrusions being separated by at least one ofthe cavities.
 6. The fuel cell of claim 5, wherein the porous siliconlayer has a non-planar surface, each of the first and second catalystlayers, the electrolyte layer and the first and second conductive layershas a non-planar configuration that conforms to the non-planar surface.7. A fuel cell, comprising: a non-porous support wafer fabricated from asilicon wafer, the non-porous support wafer including a porous siliconlayer on a first surface of the silicon wafer and a plurality of poroussilicon pillars extending from the porous silicon layer and through thenon-porous support wafer to a second surface of the non-porous supportwafer opposite the first surface, the porous silicon layer adapted toreceive a first gas presented to the second surface of the non-poroussupport wafer through the porous silicon pillars, the porous siliconlayer having a first surface area and the second surface having a secondsurface area that is less than the first surface area; first and secondcatalyst layers, the first catalyst layer adapted to be connected to ananode collector and the second catalyst layer adapted to be connected toa cathode collector; an electrolyte layer disposed between the first andsecond catalyst layers; a first conductive layer disposed between theporous silicone layer and the first catalyst layer, the first conductivelayer having a plurality of through openings adapted to allow passage ofthe first gas from the porous silicone pillars and the porous siliconelayer to the first catalyst layer; and a second conductive layerdisposed on the second catalyst layer, the second conductive layerhaving a plurality of through openings adapted to allow passage of asecond gas to the second catalyst layer.
 8. The fuel cell of claim 7,wherein the porous silicon layer has a non-planar surface and the secondsurface of the support wafer is planar.
 9. The fuel cell of claim 8,wherein each of the first and second catalyst layers, the electrolytelayer and the first and second conductive layers has a non-planarconfiguration that conforms to the non-planar surface of the poroussilicon layer.